Particulate filters are used in internal combustion engines, and especially diesel engines, to capture soot or ash present in the exhaust gas of the engine. The filters typically comprise a number of elongate cells which are generally aligned with the direction of the flow of exhaust gas when the filter is in use. The cells are open at one end and closed at the other end so that the gas may flow through and out of the cells but the soot and ash will be captured on the cell walls and remain in the filter. Adjacent cells usually face in opposing directions, with one cell open to the exhaust inlet side of the filter and the adjacent cell open to the exhaust outlet side of the filter.
In order to maintain their filtration performance some diesel particulate filters have a degree of autoselective regeneration, where the filter is regularly cleaned automatically whilst still in place within the exhaust system. There are numerous filter regeneration methods such as, for example, electrical discharge regeneration or gas discharge regeneration using nitrogen oxide. Whilst the regeneration of the filter in situ removes the majority of the soot captured in the filter cells, the performance of the filter will still diminish over a period of prolonged use, since the ash component in the filter is not reducible with regeneration. It is therefore important for the filter to be removed and cleaned as part of regular engine servicing procedures. It is equally important that a proper inspection of the cleaned filter is then carried out before the filter is put back into service, otherwise the filter performance may diminish to an undesirable level before the next scheduled engine service is reached.
Given the relatively small diameter of the cells in the filter it is difficult to access the cells and carry out a proper inspection. One invasive inspection technique uses a borescope to view the interior of each cell in order to determine cleanliness. However, borescopes are expensive and also require servicing personnel to be trained in order to operate them properly. Alternative, non-invasive inspection methods have also been devised. One such method is to measure the air pressure drop across the cleaned filter and compare it to a base value for a brand new filter. However, given the extremely low density of the ash particles it is very difficult to accurately establish whether deposits remain in the cleaned filter using this method. Another non-invasive method uses ultrasound waves which are directed into the filter cells and then a reading is taken of the waves reflected from the ash deposits lying in the cells. However, as the ash in the cells does not reflect the ultrasound waves well, the resultant ultrasound image presented to the user is usually unclear and unhelpful in determining whether ash is still present in the cleaned filter or not. In addition, ash which sticks to the longitudinal walls of the cells tends to disrupt the signal reflected from the main deposit against the end wall, which again presents an unclear image to the operator.